Digital magnetic reproducing apparatus and digital magnetic recording/reproducing apparatus employing detection of second harmonic distortion and bias current control

ABSTRACT

A digital magnetic reproducing apparatus comprises a reproducing head to reproduce data from a magnetic recording medium; an equalizer for shaping, by a partial response method, the waveform of the reproduced signal outputted from the reproducing head; and a decoder for decoding, by a maximum likelihood decoding method, the equalized reproduced signal obtained from the equalizer. The reproducing head employed in this apparatus is an MR (magnetoresistance effect) head which is capable of reducing the second harmonic distortion of the reproduced signal to be -25 dB or lower. Therefore the deterioration of the bit error rate that may result from the nonlinear distortion of the MR head can be further lowered within a sufficiently suppressed range in practical use.

This is a division of application Ser. No. 08/284,238, filed Aug. 2,1994, abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a digital magnetic reproducingapparatus and a digital magnetic recording/reproducing apparatus using,for example, a magnetic disk as a recording medium.

In the field of digital magnetic recording, there is currently inprogress the introduction of a signal processing method which ensures asatisfactory precision of code identification even in a high lineardensity area with a high-sensitivity magnetoresistance effect head(hereinafter referred to as an MR head) capable of achieving an adequatesignal-to-noise ratio even in a narrow track, i.e., a processing methodtermed PRML (Partial Response Maximum Likelihood) which is a combinationof partial response equalization and maximum likelihood decoding.

However, in realization of high surface density recording by thecombination mentioned above, there exists the problem in that a dataerror rate in a channel is deteriorated due to the harmful influence ofnonlinear distortion peculiar to an MR head. Such deterioration resultsfrom the fact that the PRML method is premised, in principle, on alinear recording/reproducing channel.

SUMMARY OF THE INVENTION

The present invention has been accomplished in view of the problemsmentioned. And an object of the invention is to provide a digitalmagnetic recording apparatus and a digital magneticrecording/reproducing apparatus which are capable of suppressing thedeterioration of the data error rate to a practically sufficient rangein a digital magnetic reproduction system based on a combination of thePRML method with a reproducing head where the second harmonic distortionof a reproduced signal is adjustable with a bias current.

According to a first aspect of the present invention, there is provideda digital magnetic reproducing apparatus comprising a reproducing headto reproduce data from a magnetic recording medium; an equalizer forshaping, by a partial response method, the waveform of the reproducedsignal output from the reproducing head; and a decoder for decoding, bya maximum likelihood decoding method, the equalized reproduced signalobtained from the equalizer. In this apparatus, the reproducing heademployed is a magnetic head which is capable of reducing the secondharmonic distortion of the reproduced signal to be -25 dB or lower.

According to a second aspect of the present invention, the apparatusfurther comprises a distortion detector for measuring the secondharmonic distortion of the reproduced signal, and a bias current controlcircuit for controlling a bias current, which flows in the reproducinghead, in such a manner as to reduce the second harmonic distortion to be-25 dB or lower.

According to a third aspect of the invention, the apparatus furthercomprises a decision circuit for making a decision as to whether or notthe second harmonic distortion is -25 dB or lower; a measuring circuitfor measuring the signal-to-noise ratio of the reproduced signal; and abias current control circuit supplied with the result outputs of boththe decision circuit and the measuring circuit, and controlling the biascurrent in the reproducing head in such a manner that thesignal-to-noise ratio becomes the highest in a range where the secondharmonic distortion of the reproduced signal is -25 dB or lower.

According to a fourth aspect of the invention, there is provided adigital magnetic recording/reproducing apparatus consisting of areproducing section and a recording section. The reproducing sectioncomprises a reproducing head to reproduce data from a magnetic recordingmedium, an equalizer for shaping, by a partial response method, thewaveform of the reproduced signal outputted from the reproducing head;and a decoder for decoding, by a maximum likelihood decoding method, theequalized reproduced signal obtained from the equalizer. And therecording section comprises a clock generator circuit for extracting aclock component from the output signal of the reproducing head andgenerating a reference clock signal from the extracted clock component;a delay circuit for delaying record data with respect to the referenceclock signal; and a recording head for recording the delayed record datafrom the delay circuit on a magnetic recording medium. In thisapparatus, the reproducing head employed is a magnetic head which iscapable of reducing the second harmonic distortion of the reproducedsignal to be -25 dB or lower.

According to a fifth aspect of the invention, the above apparatusfurther comprises a distortion detector for measuring the secondharmonic distortion of the reproduced signal, and a bias current controlcircuit for controlling a bias current, which flows in the reproducinghead, in such a manner as to reduce the second harmonic distortion to be-25 dB or lower.

And according to a sixth aspect of the invention, the apparatus furthercomprises a decision circuit for making a decision as to whether or notthe second harmonic distortion is lower than -25 dB or lower; ameasuring circuit for measuring the signal-to-noise ratio of thereproduced signal; and a bias current control circuit supplied with theresult outputs of both the decision circuit and the measuring circuit,and controlling the bias current in the reproducing head in such amanner that the signal-to-noise ratio becomes the highest in a rangewhere the second harmonic distortion of the reproduced signal is -25 dBor lower.

In any of the apparatus mentioned, the reproducing head employed is amagnetoresistance effect head. And the magnetic recording mediumemployed therein is a magnetic disk.

Due to the use of such a reproducing head capable of reducing the secondharmonic distortion of its reproduced signal to be -25 dB or lower,deterioration of the data error rate can be suppressed within twice ascompared with the distortionless condition relative to the data decodedby a combination of the partial response waveform equalization and themaximum likelihood decoding. Consequently it becomes possible tominimize the data error rate to a practically permissible rangeregardless of the nonlinear distortion peculiar to the reproducing head.

The above and other features and advantages of the present inventionwill become apparent from the following description which will be givenwith reference to the illustrative accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically shows characteristic curves depicting the operationof an MR head;

FIG. 2 graphically shows characteristic curves depicting the dependencyof a second harmonic distortion on a bias magnetic field;

FIG. 3 graphically shows characteristic curves depicting the dependencyof a bit error rate on a second harmonic distortion;

FIG. 4 is a block diagram of a first embodiment representing the digitalmagnetic reproducing apparatus of the present invention;

FIG. 5 is a timing chart of signals for explaining the operation of thedigital magnetic reproducing apparatus;

FIG. 6 is a block diagram of a second embodiment representing thedigital magnetic reproducing apparatus of the invention;

FIG. 7 is a block diagram of an exemplary circuit of a second harmonicdistortion detector;

FIG. 8 is a block diagram of an exemplary circuit of a minimum pointdetector;

FIG. 9 is a block diagram of a third embodiment representing the digitalmagnetic reproducing apparatus of the invention; and

FIG. 10 is a block diagram of an exemplary circuit of a maximum valuedetector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

(1) Conditions requisite for MR head

A general MR head has such magnetic field-to-electric resistanceconversion characteristic such as that shown in FIG. 1. The MR headoperates to convert a signal magnetic field HS into a reproduced voltageVMR centering at its operation point preset by a DC bias magnetic fieldHB. However, since the magnetic field-to-electric resistance conversioncharacteristic is nonlinear, the reproduced voltage VMR is prone to havea distortion of nonsymmetry with respect to the horizontal center line.As the nonlinear distortion causes deterioration of the data error rate,it is necessary to accurately evaluate such nonlinear distortion.

In this embodiment, a second harmonic distortion (SHD) is used forevaluation of such nonlinear distortion. The second harmonic distortionis a value defined by the amplitude ratio of the fundamental wavecomponent of the reproduced signal and the second harmonic componentgenerated due to a nonlinear response of the head. The advantageachieved by evaluation of the second harmonic distortion resides in thepoint that the nonlinear distortion caused by the MR head can beobserved substantially as a fixed value regardless of the frequencycharacteristic of the head or the measuring frequency thereof.

In checking the dependency of the data error rate on the second harmonicdistortion, it is necessary to change the second harmonic distortion ofthe reproduced waveform to be synthesized. If the second harmonicdistortion is acquired by calculation on the basis of the MR operationcurve, the distortion can be signified as a function of the biasmagnetic field EB and the signal magnetic field amplitude HO. Suchdependency is graphically shown in FIG. 2.

FIG. 3 shows the relationship between the second harmonic distortion andthe bit error rate calculated with regard to the signal magnetic fieldamplitude HO (=5 Oe) and the signal-to-noise ratio of the signalmagnetic field HS(t) used as a parameter. As expected, the bit errorrate deteriorates in accordance with an increase of the second harmonicdistortion. The deterioration is more significant in an area where thesecond harmonic distortion is higher than -20 dB. This characteristicappears to have a substantially fixed tendency regardless of thesignal-to-noise ratio. Consequently, for suppressing the deteriorationof the bit error rate to be twice the value in the distortionless stateby realizing a practically permissible bit error rate (10⁻⁶ or under),it is necessary to reduce the second harmonic distortion to be -25 dB orlower.

(2) First embodiment

FIG. 4 is a block diagram of a first embodiment representing a diskapparatus with a reproducing head 3A used as an MR head which satisfiesthe above-described conditions.

It is assumed here that the disk apparatus 1 in this embodiment is ofthe type operated by an external synchronizing system (sample servosystem). More specifically, according to this system, clock patterns forgenerating clock pulses are formed in advance radially and successivelyon a magnetic disk 2 which is driven to rotate at a constant angularvelocity (CAV) by a spindle motor (not shown), and the operation isperformed on the basis of a signal produced from such clock patterns.

The clock patterns are formed by DC magnetization unidirectionally at100 to 1000 positions per rotation to generate high-precision clockpulses.

The head 3A reproduces signals corresponding to data recorded in datasegments and also signals corresponding to the clock patterns, andsupplies such reproduced signals via a reproducing amplifier 4 to aclock generator circuit 5, an equalizer 6 and a decoder 7.

This embodiment employs an MR head adapted for satisfying theaforementioned conditions, i.e., the second harmonic distortion of thereproduced signal is -25 dB or lower. The reproducing head 3A ispositionally spaced apart by a distance L in its moving direction fromthe recording head 3B, thereby constituting a head 3 which has separaterecording and reproducing functions.

The clock generator circuit 5 comprising a PLL (phase locked loop) andso forth generates clock pulses from the reproduced signalscorresponding to the clock patterns and then supplies such clock pulsesto the individual circuits. FIG. 5 is a timing chart showing how suchclock pulses are generated.

When the reproducing head 3A reproduces the clock patterns DC-magnetizedin one direction (indicated by arrows) as shown in FIG. 5(A), signals ofisolated waveforms are reproduced by the leading and trailing edges ofthe clock patterns as shown in FIG. 5(B).

The clock generator circuit 5 recognizes, as regular clock patterns, theisolated waveforms appearing in the period during which a clock gatesignal is existent, and renews the phase of the PLL circuit in such amanner that, as shown in FIG. 5(C), the leading edge of each clock pulseis synchronized with the peak of the isolated waveform corresponding tothe leading edge, thereby generating clock pulses which are phase-lockedto the clock patterns 3.

A timing generator 8 is provided for generating a clock gate signalrequired for production of such clock pulses. The timing generator 8counts the clock pulses supplied thereto from the clock generatorcircuit 5 and, on the basis of the past hysteresis, predicts the periodof appearance of the reproduced signal corresponding to the clockpattern. The clock gate signal indicative of such period is supplied tothe clock generator circuit 5. The timing generator circuit 8 furtherserves to produce a switching signal which is used for switching arecording mode and a reproduction mode as shown in FIG. 5(D).

First the operation in the reproduction mode will be described below. Inthis mode, the equalizer 6 and the decoder 7 discriminate the reproducedsignal with reference to the rise time (hereinafter referred to as dataexistent phase) of the clock pulse, and decode the reproduced signal tothereby reproduce the data recorded on the magnetic disk 2.

In this embodiment, the art of partial response equalization is adoptedfor the equalizer 6, and the art of maximum likelihood decoding for thedecoder 7. The decoder 7 reproduces the position data of the head 3 inthe radial direction of the disk on the basis of the reproduced signaland then supplies such position data to a delay time control circuit 9.

Meanwhile in the recording mode, a record data generator 10 operates inresponse to clock pulses. More specifically, the record data generator10 converts input source data into record data, which are synchronizedwith the clock pulses, by a predetermined method of modulation suitedfor recording, and then supplies the record data to both a pulse delaycircuit 11 and the delay time control circuit 9.

Under control of the delay time control circuit 9, the pulse delaycircuit 11 compensates for the positional deviation of the data thatresults from the distance L between the reproducing head 3A and therecording head 3B in the moving direction thereof. This compensationneeds to be performed due to the presence of a time lag between thetrack pattern (FIG. 5(E)) seen from the recording head 3B and the trackpattern (FIG. 5(A)) seen from the reproducing head 3A.

In this stage of the operation, the pulse delay circuit 11 delays therecord data (FIG. 5(G)) inputted from the record data generator 10, soas to compensate for the positional deviation in the inversion ofmagnetization caused by the time lag and also for the phase deviation(nonlinear bit shift) in the inversion of magnetization depending on thepattern of the record data, and then outputs the record data delayed asshown in FIG. 5(H). The pulse delay circuit 11 further serves to delaythe switching signal (FIG. 5(D)) inputted from the timing generator 8,thereby producing a write enable signal (FIG. 5(F)).

A recording amplifier 12 amplifies the record data thus delayed, andthen supplies to the recording head 3B a current corresponding to therecord data, whereby the data is recorded on the magnetic disk 2.

According to the construction described above, the error rate caused bythe signal processing in the decoder 7 can be suppressed to 10⁻⁶ or sodue to the use of the MR reproducing head 3A where the second harmonicdistortion of the reproduced signal can be reduced -25 dB or below.Consequently it becomes possible to obtain an improved disk apparatus 1which is capable of minimizing the deterioration of the error rate thatresults from the nonlinear distortion.

(3) Second embodiment

The first embodiment mentioned above represents a general disk apparatuswhere the second harmonic distortion in the MR reproducing head 3A is-25 dB or lower. Now a description will be given on a second embodimentrepresenting a particular disk apparatus where a bias magnetic field(bias current), which minimizes the second harmonic distortion of an MRhead capable of satisfying the above condition, is substantiallycoincident with a bias magnetic field (bias current) which enhances thesignal-to-noise ratio to the optimum 7.

In FIG. 6 where like reference numerals denote like component circuitscorresponding to those in FIG. 4, a disk reproducing apparatus 20 isshown as a whole. This disk reproducing apparatus 20 is similar inconstruction to the aforementioned first embodiment except that itfurther comprises a second harmonic distortion (SHD) detector 21 fordetecting the second harmonic distortion of a reproduced signal, and abias current control circuit 22 for controlling, on the basis of theresult of such detection, a bias current (bias voltage) supplied to areproducing head 3A.

The second harmonic distortion detector 21 is so constructed as shown inFIG. 7. In the second harmonic distortion detector 21, a reproducedsignal is inputted to two band pass filters 21A and 21B, where afundamental wave component and a second harmonic component are extractedrespectively.

The amplitude values of such signal components extracted by the bandpass filters 21A and 21B are detected by sample holding circuits 21C and21D respectively, and the ratio of the two values is calculated by adivider 21E so that the second harmonic distortion is detected.

Meanwhile the bias current control circuit 22 is so constructed as shownin FIG. 8. The bias current control circuit 22 comprises a functionalpart for detecting an optimal bias current relative to the reproducinghead 3A, and another functional part for holding an optimal biascurrent.

The bias current control circuit 22 applies a current value settingvoltage, which is outputted from a ramp waveform generator 22A, to avoltage-controlled current source 22C via a switch 22B, whereby the biascurrent is varied so that the minimum point of the second harmonicdistortion obtained during the above period can be detected by a minimumpoint detector 22D.

The minimum point detector 22D is so constructed that the secondharmonic distortion obtained from the reproduced signal is compared by acomparator 22D3 with the value stored in a memory 22D2. When the resultof a decision signifies that the second harmonic distortion obtainedfrom the reproduced signal is smaller than the value stored in thememory 22D2, a switch in a switching circuit 22D1 is turned to itswhite-circle side in FIG. 8, whereby the value stored in the memory 22D2is updated to the new value.

The minimum point detector 22D outputs a control signal to the sampleholding circuit 22E every time the minimum value of the second harmonicdistortion is updated, and holds the current-value setting voltage atthat instant. More specifically, the value held in the sample holdingcircuit 22E is continuously updated until the second harmonic distortionof the reproduced signal passes through the minimum point.

During a search for the current-value setting voltage which applies anoptimal bias current, a switch in a switching circuit 22B is connectedto its white-circle side in FIG. 8 and, after completion of scanning theramp waveform, the switch is turned to its black-circle side, wherebythe second harmonic distortion during the operation of the diskapparatus 1 is maintained in the optimal state.

Since the bias current suited for minimizing the second harmonicdistortion of the reproducing head 3A employed in this embodiment issubstantially coincident with the bias current for enhancing thesignal-to-noise ratio to the best, the signal-to-noise ratio is renderedmaximum relative to the reproduced signal from the head 3A where suchset bias current flows, hence realizing full utilization of theperformance of the magnetic disk reproducing apparatus to consequentlylower the error rate.

(4) Third embodiment

The foregoing second embodiment is premised on that the bias magneticfield (bias current), which is suited for minimizing the second harmonicdistortion of the MR head where the second harmonic distortion can belowered -25 dB or below, is substantially coincident with the biasmagnetic field (bias current) for enhancing the signal-to-noise ratio tothe best. In contrast therewith, the third embodiment represents anotherexemplary disk apparatus where an optimal bias current is settable whensuch coincidence is not ensured.

In FIG. 9 where like reference numerals denote like component circuitscorresponding to those shown in FIG. 6, a disk apparatus 30 furthercomprises a decision circuit 31 for making a decision as to whether theaforementioned condition of prescribing the second harmonic distortionto -25 dB or less is satisfied or no., and an S/N measuring circuit 32.In this third embodiment, a bias current control circuit 33 shown inFIG. 10 is employed for setting an optimal bias current on the basis ofthe result of the decision relative to the distortion level and theresult of measuring the signal-to-noise ratio.

As shown in FIG. 10 where like reference numerals denote like componentcircuits corresponding to those in FIG. 8, the bias current controlcircuit 33 includes a maximum value detector 33A for detecting the bestpoint of the signal-to-noise ratio, in place of the aforementionedminimum point detector 22D which is used for detection of the minimumpoint relative to the second harmonic distortion, and further includesan AND circuit 33B for taking an AND logic product of the result outputof the detector 33A and that of the decision circuit 31.

The maximum value detector 33A is so formed that, only when thesignal-to-noise ratio inputted thereto is higher than the ratio storedin a memory 33A2, such ratio in the memory 33A2 is updated to the newvalue.

The AND circuit 33B is so formed that, only when the aforementionedconditions are satisfied as the second harmonic distortion obtained fromthe reproduced signal is -25 dB or lower and the signal-to-noise ratiois the highest at the present instant, a control signal is outputted torenew the current-value setting voltage held in the sample holdingcircuit 22E.

According to the construction mentioned, a bias current of an adequatevalue for achieving the highest signal-to-noise ratio within aconditional range of lowering the second harmonic distortion -25 dB orbelow can be caused to flow in the reproducing head 3A, so that even ifthe response characteristic of the reproducing head 3A has somevariation, it is still possible to realize a satisfactory disk apparatuswhich is capable of performing its operation at the minimized data errorrate.

(5) Other embodiment

Each of the above embodiments represents an exemplary disk apparatushaving both a recording section to record data on a disk and areproducing section to reproduce the data from the disk. However, it isto be noted that the present invention is not limited to suchembodiments alone and may also be applied to a type having merely areproducing section.

Moreover, although each of the above embodiments is concerned with anexemplary case of using an MR head as the reproducing head 3A, thepresent invention is widely applicable to any other case of using amagnetic head where its second harmonic distortion is adjustable by abias current.

Further in addition to the above embodiments representing a diskapparatus, the present invention is applicable also to any digitalmagnetic reproducing apparatus which employs an MR reproducing head andadopts the PRML method for signal processing.

What is claimed is:
 1. A method of adjusting a magnetic head biascurrent comprising the steps of:a) measuring a second harmonicdistortion of a reproduced signal; and b) adjusting the magnetic headbias current to maintain the second harmonic distortion at a value below-25 dB.
 2. A digital magnetic reproducing apparatus comprising areproducing head to reproduce data from a magnetic recording medium; anequalizer for shaping, by a partial response method, a waveform of areproduced signal output from said reproducing head; and a decoder fordecoding, by a maximum likelihood decoding method, an equalizedreproduced signal obtained from said equalizer;said apparatus furthercomprising: a distortion detector for measuring a second harmonicdistortion of the reproduced signal; and a bias current control circuitfor controlling a bias current, which flows in said reproducing head, toreduce the second harmonic distortion to be -25 dB or lower.
 3. Thedigital magnetic reproducing apparatus of claim 2, wherein saidreproducing head is a magnetoresistance effect head.
 4. The digitalmagnetic reproducing apparatus of claim 2, wherein said recording mediumis a magnetic disk.
 5. A digital magnetic recording/reproducingapparatus comprising :a reproducing section comprising a reproducinghead to reproduce data from a magnetic recording medium; an equalizerfor shaping, by a partial response method, a waveform of the reproducedsignal output from said reproducing head; and a decoder for decoding, bya maximum likelihood decoding method, an equalized reproduced signalobtained from said equalizer; and a recording section comprising a clockgenerator circuit for extracting a clock component from the outputsignal of said reproducing head and generating a reference clock signalfrom the extracted clock component; a delay circuit for delaying recorddata with respect to the reference clock signal; a record data generatorconnected to an output of the clock generator and a recording head forrecording the delayed record data from said delay circuit on a magneticrecording medium; said apparatus further including: a distortiondetector for measuring a second harmonic distortion of the reproducedsignal; and a bias current control circuit for controlling a biascurrent, which flows in said reproducing head, to reduce the secondharmonic distortion to be -25 dB or lower.
 6. The digital magneticreproducing apparatus of claim 5, wherein said reproducing head is amagnetoresistance effect head.
 7. The digital magnetic reproducingapparatus of claim 5, wherein said recording medium is a magnetic disk.